Bottles made of polyvinyl chloride (PVC) and polyethylene terephthalate (PET) are in widespread use as containers for beverages such as soda. Collections of such plastic containers for recycling generally include clear colorless bottles of both PET and PVC. Clear colorless reclaimed PET is substantially more valuable for reprocessing when free of PVC. However, the similarity in density (1.34 g/cc for PET, 1.32 g/cc for PVC) precludes separation of the two materials on the basis of simple density devices most common in recycling systems.
Alternative separation methods have been proposed which employ density alteration by solvent soaking or X-ray attenuation. See Grimm et al., U.S. Pat. No. 4,617,111, issued Oct. 14, 1986, and European Patent Publication No. 291,959, published Nov. 23, 1988, respectively. The X-ray attenuation method examines whole bottles or large segments before grinding. PET does not attenuate X-rays by absorption significantly, whereas PVC produces substantial attenuation. The intensity contrast between PET and PVC is used for sorting purposes. However, the X-ray technique requires use of ionizing radiation with its inherent hazards, and requires that the X-ray radiation penetrate through the plastic article.
The solvent soaking technique is performed on ground bottles and requires the use of one or more organic solvents. PET density is not affected during solvent exposure. By contrast, upon prolonged solvent exposure, the density of PVC approaches the solvent density. This provides a sufficient density difference to allow sorting with traditional density sorting devices. However, the use of organic solvents creates safety and disposal problems and is thus undesirable. Residual solvent must be removed from the treated PET before further processing.
Polarized light has been used in a variety of sorting and detection systems, for example, an identification system for ID cards (Reiner et al. U.S. Pat. No. 4,659,112, issued Apr. 21, 1987), a system for sorting translucent objects such as colored rice grains (Fraenkel U.S. Pat. No. 3,197,647, issued Jul. 27, 1965), a system using polarized infrared light to sort foreign objects from vegetables (Drabs U.S. Pat. No. 4,262,806, issued Apr. 21, 1981, a system for identifying invisible written characters (Marks U.S. Pat. No. 3,313,941, issued Apr. 11, 1967), a system for inspecting glass containers, vials and bottle rims (see Mathias U.S. Pat. No. 3,355,980 issued Dec. 5, 1967, Nicholson et al. U.S. Pat. No. 4,417,662, issued Nov. 19, 1983 and Nakatani et al. U.S. Pat. No. 3,963,348, issued Jun. 15, 1976), and a system for measuring the surface orientation of polymeric materials (Japanese Patent Publication No. 63-243836, Oct. 11, 1988). None of these patents provide a system for sorting plastic articles such as PET and PVC containers which have different degrees of crystallinity. The present invention addresses the various disadvantages of prior methods for sorting plastic articles.